Heretofore, for purposes of improving the vulcanization property of a vulcanizable rubber composition for various rubber products such as tires and the like, it is known to compound a thiuram compound such as TMTD (tetraethyl-thiuram disulfide) of the formula, ##STR3## and the like with the rubber.
Tread rubber of tires for passenger cars and the like capable of running at a high speed requires a high grip property (increasing the coefficient of friction between a road surface and a tread). Therefore, a tread rubber having a large hysteresis loss is generally used.
However, this causes a generation of a large amount of heat in the tread rubber, as a consequence, the cross-linking of the tread rubber is broken resulting in a decomposition of the rubber components.
This decomposition results in the generation of foam to form a sponge-like mass and the tire is finally broken.
In addition, the resulting high temperature reduces the modulus of elasticity and the hardness of the rubber. As a result, running stability is disadvantageously disturbed and other undesirable phenomena occur.
It is known that when the above-described TMTD, TMTM (tetraethylthiuram monosulfide) or the like is used as a vulcanization accelerator for tread rubber, the above-described problems are solved (cf. Japanese Patent Application Laid-Open No. 48739/1987).
However, the above-described conventional rubber compositions do not satisfy each of the characteristics of vulcanization speed, processing stability, heat resistance and creep resistance to a sufficient extent.
Moreover, conventional tread rubber does not sufficiently meet high speed running stability as a result of even further high speed (for example, 200 km/hr. or more) used recently.